As illustrated in FIG. 1, an ionization detector 100 typically comprises a body 102 having a first chamber 110 for generation of ionizing particles and a second chamber 120 connected to the first chamber 110 for receiving a sample gas 122. The sample gas 122 is conveyed in a carrier gas and is provided to the second chamber 120 by a conduit 130 which typically is provided in the form of a separation column. The first chamber 110 includes a source of ionizing particles (not shown), such as a radioactive source or an electrical discharge, and is typically swept by a detector gas 112 selected from the class of known noble gases. The presence of the detector gas 112 in the first chamber 110 causes ionizing particles, in the form of photons and metastables, to be produced. The flow of the detector gas 112 from the first chamber 110 to the second chamber 120 causes the ionized particles to be mixed with the sample gas 122, thus causing the sample molecules of interest, considered herein as analytes, to be ionized. The second chamber 120 includes electrodes 124,126,128 for detecting the ionized sample molecules by use of an electrometer circuit (not shown) connected to the electrodes 124,126, 128.
Detector sensitivity may be measured in a plot of detector response versus analyte concentration or analyte quantity. The range over which the detector sensitivity is constant is called the linear dynamic range, and the entire range over which the response is variable with analyte concentration or quantity is called the dynamic range of the detector. The upper limit of the dynamic range is determined when detector sensitivity falls to an unusable value, typically zero, and the detector is said to be saturated. The lower limit of the dynamic range occurs at a minimum detectable level (MDL).
Particular examples of ionization detectors include the electron capture detector and the discharge ionization detector.
Electron capture detectors for gas chromatography are well known in the art. This type of detector offers high sensitivity and high selectivity towards electrophilic compounds and is widely used for detecting trace amounts of pesticides in biological systems and in food products. Such compounds typically contain halogens which combine with free electrons that are created in the ionization chamber in the detector. The resulting decrease in free electrons in the ionization cell is monitored as an indication of the concentration of the compounds in a sample.
A discharge ionization detector operates by applying a high voltage across discharge electrodes that are located in a gas-filled source chamber. In the presence of a detector gas such as helium, a characteristic discharge emission of photons occurs. The photons irradiate an ionization chamber receiving a sample gas that contains an analyte of interest. Ions are produced in the ionization chamber as a result of photon interaction with ionizable molecules in the sample gas. Helium metastables are also generated in the source chamber and are found to play a role in ionization of the analyte of interest.
FIG. 2 illustrates a linearity plot 200 that is typical of the dynamic range of a helium discharge ionization detector. The magnitude of the ionized analyte molecules is manifested as a current that can be measured to ascertain the composition of one or more analytes. In the illustrated linearity plot, the analyte is carbon-12 (C.sub.12). The response factor should ideally be constant (in other words, flat) irrespective of the amount of the analyte introduced into the detector. As illustrated, the response factor is flat over the linear dynamic range 210 but decreases in a second region 220 when higher amounts of analyte are introduced to the detector.
Although the design of ionization detectors continues to be an object of study in the prior art, there nonetheless exists a need for an ionization detector having a detector response that exhibits greater linear dynamic range.